In automation and process engineering frequently sensors or measuring devices are used, which convert detected measurement values—such as pressure, temperature, flow rate as well as distance and vibration—into an output signal representing this measurement value in the form of an analog current or voltage signal and provide this signal at their cable port or plug connection for further processing, such as a PLC. In order to enable that these signals can be evaluated, they previously have to be amplified, which usually takes place in a driver stage, which is arranged as a separate or combined chip on the circuit board in the sensor housing. Such a chip is, for example, the device AD5410 from Analog Devices.
However, these output stages can easily be destroyed in the event of an incorrect connection. The ESD protection diodes obligatory integrated at the driver output act as intended against voltage transients, which the driver output can be exposed even during normal handling by being led out from the sensor. If, however, the negative voltage supply connection (=ground) is connected to the positive supply voltage (UB+) and the output terminal is connected to the negative supply voltage (UB−), a short-circuit current flows through the ESD protection diode which is connected between ground and driver output. Moreover, measuring devices with high operating current consumption can be destroyed if the negative supply connection is not connected, since the entire operating current of the device flows through the ESD protection diode and simulates a proper connection by the thus seemingly correct function, until the ESD protection diode is destroyed due to overload.
A possible solution envisages to connect a sufficiently powerful protection diode in parallel to the analog output against ground and to provide a resistor between the output stage and the actual signal output which limits the current in case of short-circuit. To this end, the resistance must not be particularly highly resistive such as not to increase too much the resulting voltage drop in normal operation, however, it must have a certain performance class in order to withstand the heat generated by the short-circuit current. However, such devices are large and expensive, such that their installation is difficult and uneconomical.
From US 2007/0145484 A1 it is known to achieve ESD protection by a protection transistor that is connected in parallel with the output stage transistor. The US 2009/0141412 A1 reference relates to an interface, where the interface system is used to connect an electrical device to an electrical bus. By means of a special circuit current is prevented from flowing from the electrical device to the electrical bus when the power source is disconnected from the electrical bus.